1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus, plotter or similar image forming apparatus and more particularly to a toner content control system for an image forming apparatus of the type using a two-component type developer, i.e., a toner and carrier mixture.
2. Description of the Background Art
Accurate control over the toner content of a two-component type developer is one of prerequisites for an electrophotograpahic image forming apparatus of the type using such a developer. Japanese Patent Laid-Open Publication No. 1-154179, for example, discloses a toner content control system of the type maintaining replenishing fresh toner in accordance with the output of a toner content sensor. It has been customary with this type of toner content control system to add some circuit for reducing irregularity in the output of the toner content sensor, thereby confining the output of the toner content sensor in a preselected range, as taught in Japanese Patent Laid-Open Publication No. 63-58472.
However, the prior art toner content control system stated above has the following problem left unsolved. The density of an image formed by an electrophotographic image forming apparatus varies in accordance with the ambient temperature and humidity as well as developing conditions even when the toner content remains constant. This problem, which is extremely substantial, is ascribable to the fact that electrophotography develops a latent image by frictional electrification. To solve this problem, it is a common practice to form a reference pattern on a photoconductive element and set a target toner content and replenish toner in such a manner as to maintain the density of the reference pattern constant.
Generally, toner formed of denatured polyester resin is advantageous over toner formed by suspension polymerization in that its shape can be easily controlled and is therefore feasible for blade type cleaning. Also, such toner is free from the problem of toner formed by emulsion polymerization that a surfactant remains in toner or contaminates a photoconductive element and a developing roller, obstructing the frictional electrification of the toner.
However, a problem with toner formed by denatured polyester resin is that toner grains themselves are charged to weak positive polarity opposite to usual charge polarity and are therefore apt to decrease in charge when left unused over a substantial period of time. Another problem is that the toner must be agitated for a longer period of time than conventional toner to be fully charged after replenishment. An image forming apparatus of the type using the toner formed of denatured polyester resin and the toner content control forming a reference pattern has some problems that will be described specifically later.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2003-56508, 2002-251063, 10-186830, 9-90734 and 10-142935.